The conventional method for determining the radioactivity of .beta.-ray emitters includes the Gas-Flow Counter Method and the Liquid Scintillation Counter (LSC) Method and the like and in particular, the LSC Method can determine the radioactivity of the low energy .beta.-ray emitters with a high sensitivity and thus this method has been widely used. In the LSC Method, the samples are dissolved into a liquid scintillator such as toluene and the like and they are measured for each sample. A special vial and a liquid scintillator of from several milliliters to 10 milliliters are used for each sample to be measured. Therefore, a large scaled equipment and a great deal of labor are required for treating the waste radioactive liquid of the samples after counting. In addition, it is actually impossible to recover the samples used for the measurement of the .beta.-ray emitters in the LSC Method.
In order to overcome the above mentioned disadvantages in the LSC method, Japanese Patent Application No. 354668/92 was filed by the same applicant, S. Baba as that of the present application. It discloses a method and an apparatus for determining radioactivity, comprising the steps of applying a plurality of liquid samples onto a number of flat-bottomed wells provided in a plastic microplate, drying the samples to provide measuring samples, facing the dried samples to a radiation recording plate, allowing the samples to stand for a prescribed period to record the radiation from the samples, and measuring the radioactivity recorded in the radiation recording plate from the samples.
The maximum range of the .beta.-rays radiated from tritium is 0.6 mg/cm.sup.2. Air and helium have the density of 1.28 mg/cm.sup.3 and 0.18 mg/cm.sup.3, respectively at standard conditions. Thus, the maximum ranges of the .beta.-rays radiated from the tritium under standard conditions in the air and helium are about 5 mm and about 33 mm, respectively.
In the conventional method for determining the radioactivity, the microplate and radioactivity recording plate are faced each other under one atmospheric pressure in air, so that the .beta.-rays radiated from the samples disposed in the bottom of the wells can not reach to the radiation recording plate or can reach slightly because the .beta.-rays are absorbed by the air present between the the samples to be determined and the radiation recording plate. Further, the thin (for example, 0.5 mg/cm.sup.2 thickness) plastic film has been disposed between the microplate and the radiation recording plate in the conventional method. However, such a plastic film can not be disposed therebetween in the determination of tritium.